Ever since the emergence of high tech. solid state technology, attempts have been made to develop high resolution, low dosage X-ray imaging devices. Ideally, such a device would allow a high resolution image to be viewed in real time at a location well removed from the X-ray source while at the same time subjecting the object (or person) under scrutiny to a relatively low dose of X-radiation.
In FIG. 1 we illustrate what happens when a quantum of X-radiation 2 enters a solid material 1 such as a crystal of silicon or germanium. The X-ray will gradually dissipate over a distance proportional to its energy. The energy dissipated per unit length is referred to as the stopping power of the material, being 1.6 keV/micron for silicon and 3.2 keV/micron for germanium. Thus the range of 100 keV X-rays is about 60 microns in silicon and about 30 microns in germanium.
Two mechanisms are involved in dissipating the X-ray. First is the Compton effect which is a momentum exchange between the quantum and a free electron in the solid resulting in an increase in wavelength for the former and an increase in energy for the latter. Second is the photoelectric effect which is the ejection of an inner shell electron converting it, at least temporarily, to a free electron. The net effect of the two mechanisms combined is to temporarily increase the conductivity of the irradiated material in a region immediately surrounding the trajectory of the X-ray. The higher the stopping power of the detector material the smaller the volume of this region.
This effect of the X-radiation is the basis for a typical X-ray detector of the prior art such as the one shown in schematic form in FIG. 2. Block 21 of silicon or germanium is divided into three parts. In the center is the detecting region 22 which is of intrinsic material (typically formed through lithium doping), while the two ends that connect to leads 23 and 24 are of P and N type material. In the absence of X-radiation 2 the resistance between 23 and 24 is very high (so there is negligible leakage current) both because of the intrinsic resistivity of 22 and because of the opposite conductivity types at the ends. Once radiation is applied to 21, large numbers of electrons and holes are formed and the resistance between 23 and 24 drops so that a substantial leakage current can be measured.
The main disadvantage of this type of detector is that a relatively large volume of semiconductor material, of the order of 10 cc., is required for adequate sensitivity. If smaller volumes are used, only some of the electrons formed in the wake of the X-ray will be available for collection and sensitivity is reduced. Thus detectors of this type are unsuitable for use in imagers as they provide poor resolution.
Because of its complete compatibilty with integrated circuits, silicon would be the preferred choice for an X-ray imager but, as discussed above, its low stopping power prevents good pixellation, at least with the PiN structures known to the prior art. Germanium has a larger stopping power for X-rays but it is still relatively small and, also, the intrinsic resistivity of germanium at room temperature is quite low (about 50 ohm-cm.). An example of a material that has large stopping power is cadmium zinc telluride (CZT) but this material is not available in crystalline form. This results in low carrier mobility and short minority carrier lifetime, leading to poor signal/noise ratio and slow response time. Similar problems apply to mercury iodide which suffers from the additional serious drawback of being extremely soft.
Because of the difficulties discussed above, the number of X-ray imager designs described in the prior art is quite small. References that we have found to be of interest include Antonuk et al. (U.S. Pat. No. 5,079,426 January 1992) who describe an array of photoconductors made of hydrogenated amorphous silicon. Snoeys et al. (U.S. Pat. No. 5,237,197 August 1993) describe an array of PiN diodes which are biassed to collect charge generated by ionizing radiation. The junctions are parallel to the wafer surface so occupy a fair amount of space. Lee (U.S. Pat. No. 5,652,430 July 1997) describes a structure that also uses a photocondcutor as the transducer. A charge accumulating capacitor is included with each detecting element for periodic discharge into a display.